Changes in polymorphic gene frequencies in strains of chickens selected for resistance to Marek's disease.

Resistance to Marek's disease (MD) was selected over five generations in three strains and their four substrains. 2. MD mortality decreased consistently on natural exposure, but in birds artificially inoculated the mortality increased again in later generations. 3. Changes of gene frequencies at five polymorphic loci--blood group A, B and D, plasma esterase and alkaline phosphatase loci--were examined. 4. The frequencies of blood group A6 and B8 alleles showed a consistent trend of increase and the B11 allele gradually decreased in most strains. The Akp allele of alkaline phosphatase increased significantly in two of seven strains. 5. Comparisons of homozygotes and heterozygotes revealed a significant difference only at the A locus.